PW Doppler ultrasound is used in medical ultrasound exams to examine blood flood and measure blood flow velocity. FIG. 1 is a schematic depiction illustrating the application of PW Doppler ultrasound to measure blood flow velocity. A blood vessel 12 contains blood flowing in the direction indicated by arrow 14. An ultrasound probe 16 transmits a series of ultrasound pulses along a scan line 17. Pulses are reflected back to ultrasound probe 16 by structures along scan line 17. Signals reflected from structures at a particular depth may be identified by time-gating the registration of reflected pulses received at probe 16. Transmitted pulses incident on sample volume 18 within blood vessel 12 are reflected by erythrocytes moving in the flowing blood. Because the transmitted pulses interact with moving blood components, such as erythrocytes, the frequency of the reflected pulses will differ from the frequency of the transmitted pulses. This change in frequency is known as a Doppler shift. After determining the frequency of the pulses received from sample volume 18, the velocity of blood flowing through the target volume may be calculated from the frequency shift using the Doppler equation
  V  =                    F        D            ⁢      c              2      ⁢              f        0            where V is the velocity in the sample volume, FD is the Doppler shift, c is the velocity of sound in blood, and f0 is the transmitted frequency.
Because scan line 17 is at an angle to the direction of the blood flow, the Doppler shift indicates only the component of blood flow velocity parallel to scan line 17. To obtain the true blood flow velocity, the measured velocity value can be angle-corrected for the angle θ between scan line 17 and the direction of blood flow 14. This may be achieved, for example, by dividing the measured velocity value by the cosine of the angle θ between the scan line 17 and the blood flow direction 14.
In conventional ultrasound Doppler exams, angle correction is based on an estimate of the angle between the scan line and the direction of blood flow. Typically, a human sonographer estimates the direction of blood flow from the shape and configuration of anatomical structures on a B-mode image, and manually places a cursor on the image to indicate the blood flow direction. The direction of blood flow is usually assumed to be parallel to the walls of the blood vessel. This method of manual angle estimation may introduce error in velocity measurement since blood flow direction is not necessarily parallel to the blood vessel wall. The error may be enlarged when using a large correction angle (e.g., an angle greater than 60°) since small changes to such a large angle result in a comparatively large difference in the value of the cosine of the angle. To minimize error in manual angle estimation, sonographers must position the ultrasound probe manually to reduce the angle between the ultrasound beam and the blood flow, which is time consuming and requires experience. In some cases, repeated velocity measurements (preferably with different angles) may be made to gain a feel for the variability of measurements. Even with repetitive measurements, the principal blood flow direction may not be found since the haemodynamics of the blood flow may be complex for reasons including helical, converging, diverging and/or turbulent flow patterns; bleeding (blood passing through the vessel wall); atherosclerosis (changes in blood vessel shape); stenosis (narrowing of blood vessels); complex blood vessel geometry; and changing blood flow direction over time.
In addition, when the blood flow direction is perpendicular to the ultrasound beam direction, no Doppler frequency shift will be detected. This is problematic in cases where the sample volume spans a helical flow, in which cases there are Doppler shifts in a wide range of directions. In addition, because of the finite size of the Doppler sample volume, transit time spectral broadening may produce bidirectional Doppler shifts.
Several approaches using ultrasound pulses transmitted and/or received at several different directions, known as vector Doppler measurement, have been proposed to improve the accuracy of the blood flow velocity measurement. Since the Doppler pulses corresponding to each direction can be used to estimate the velocity component along that direction (the bisector of the angle between each transmit direction and corresponding receive direction), an estimate of blood flow velocity in an arbitrary direction can be reconstructed from the components of the velocities along the measured directions. Sample publications describing the principles and implementation of vector Doppler measurement include the following:
U.S. Pat. No. 4,622,977, Namekawa et al.
U.S. Pat. No. 5,701,898, Adam et al.
U.S. Pat. No. 5,724,974, Goodsell, Jr. et al.
U.S. Pat. No. 5,409,010, Beach
U.S. Pat. No. 5,910,119, Lin
U.S. Pat. No. 6,464,637, Criton et al.
U.S. Pat. No. 6,589,179, Criton et al.
US Patent Application no. 2007/0073153, Tortoli
Since vector Doppler approaches differ from conventional Doppler ultrasound, implementing these approaches in a manner that is intuitive and easily learned by sonographers is a challenge. A related challenge is presenting results of vector Doppler ultrasound measurements in a way that is readily intelligible and useful for clinical purposes. There is accordingly a need for vector Doppler ultrasound systems that provide improved accuracy of velocity measurement and other haematological measurements, are intuitive and easily learned by sonographers, and provide interfaces that present results of vector Doppler ultrasound measurements in a readily intelligible and clinically useful fashion.
One particular application of Doppler ultrasound is the study of blood flow turbulence, particularly in regions of actual or suspected plaque deposits and/or stenoses. Information about blood flow turbulence may be used to evaluate the risk of blood vessel trauma, such as plaque rupture, which are known to cause stroke and myocardial infarction. Blood flow turbulence has previously been estimated by reconstructing the velocity components perpendicular to the vessel axis, on the assumption that flows perpendicular to the vessel axis indicate turbulence. These methods work well when the vessel wall is parallel to the vessel axis. However, plaque deposits may introduce local variations into the shape of the vessel wall that change flow direction but do not cause turbulent flow. There is accordingly a need for ultrasound Doppler systems that provide a better method for evaluating blood flow turbulence.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.